This section provides background information related to the present disclosure, which is not necessarily the conventional technology.
The explosive growth of user data improves the requirement for a data transmission rate and transmission efficiency. With increasing communication load between a user and a base station, direct communication is performed between geographical adjacent devices without relay of the base station, in this case, not only load of the base station can be reduced but also a signal is good due to a small communication distance. Furthermore, transmitting power between the devices may be low, which is beneficial to reduce communication interference to other devices. D2D communication technology is developed under such background.
The D2D communication changes the conventional user-to-base-station communication mode, a part of functions of the base station are conveyed to a user equipment, hence, it is a challenge to design protocols for both a physical layer, an MAC (Media Access Control) layer and a high layer.